It is known to control the speed of a locomotive by controlling the locomotive's diesel engine throttles and main generator excitation voltage. In diesel locomotives, the diesel engine turns the main generator and the generator supplies electrical power to the electric traction motors. The amount of tractive effort supplied by the tractive motors is determined by the engine speed of the diesel engines and the excitation of the main generator. Locomotive engine speed is discretely controlled in a series of throttle settings. Each engine speed, or throttle setting, provides a fixed amount of power to the generator, characterized by a sudden increase or decrease in power when the throttle setting is changed. For each throttle setting, a predefined excitation voltage is provided to the main generator that in turn determines the tractive effort supplied by the tractive motors. Examples of such systems are described in U.S. Pat. Nos. 5,629,567; 4,602,335; 4,234,922; and 4,118,774.
It is also known in the art to provide distributed power control system for locomotives (Distributed Power or DP), in which the operation of one or more remote locomotives, or group of locomotives in a train consist, is remotely controlled from a lead locomotive of the train via a hard-wired radio communication link. One such Radio based DP is commercially available under the trade designation Locotrol radius from the General Electric Company and is described in GE's U.S. Pat. No. 4,582,280. Hard wired systems have been available for over 20 years from various vendors and provide communication between locomotives directly connected mechanically together to form a consist and connected electrically via Multiple-Unit (MU) cables to provide intra-consist communication. Typically, MU cables include a dedicated throttle train line to allow remote control of locomotives throttle settings by electrical signals transmitted from a control locomotive to a controlled locomotive along the line. Despite having the capability to communicate with other locomotives, some locomotives are not equipped with the required equipment or modifications to allow remote control of the excitation of the locomotive's main generator and consequently, the locomotives cannot be used in slow speed operations.